1 How hot is it today? How cold was it last night? Thermometers tell us the temperature. They can be used indoors or outside. The word "thermometer" comes from two Greek words. "Therm-" means heat, and "-meter" means to measure. A thermometer is a tool to measure heat.2 Many thermometers are clear tubes. The tubes are sealed at both ends. They have a colored liquid inside. There is a scale with numbers along the side of the tube. The colored liquid moves up in the tube when it is warmer. The colored liquid moves down in the tube when it is colder.3 The colored liquid is often alcohol. Alcohol expands, or takes up more space, when it heats up. It takes up less space, or contracts, when it cools. A dye, usually red, is used to color the alcohol. It makes it easier for us to read. Some thermometers have mercury inside the tube. It works better than alcohol for measuring higher temperatures.